1. Field of the Invention
The present invention generally relates to a load controlling system used in a packet communication exchange. More specifically, the invention is directed to a load controlling method employed in a packet communication exchange, capable of accessing a data bus with high efficiency.
2. Description of the Related Art
In a known packet transmission exchange, data packets are transmitted and/or received between a plurality of modules connected to a common communication medium, for instance, a data bus. When a data packet transmission demand for the common communication medium causes contention disputes, contention control is carried out so as to apply a transmission right to a specific module among those modules where the packet contention disputes occur. Also, another packet contention controlling method has been proposed, in consideration of an equality, where such a transmission right is not applied eccentrically to a specific module.
The above-described conventional contention controls of the packet communication system are known from, e.g., Japanese Laid-open (KOKAI) patent application No. 59-200554 (1984) corresponding to U.S. patent application Ser. No. 485034 filed on Apr. 13, 1984 in the U.S.A. by Anthony Acampora, entitled "Network Protocol For Integrating Synchronous and Asynchronous Traffic On A Common Serial Data Bus," now U.S. Pat. No. 5,040,170.
In accordance with these conventional packet contention controlling systems, the packet transmission delays occurring between the modules employed in the packet communication exchange can be controlled. However, another packet transmission delay occurring in the buffer circuit employed in the module is rather more serious than the above-described delays caused in the modules, because of the major reason for the delay. This buffer circuit is used for packets waiting for packet transmission to the trunk line. This packet transmission delay in the buffer circuit is particularly emphasized in such a case as when the packets are temporarily concentrated in the module corresponding to the specific trunk line, which are sent from the communication medium in the pocket communication exchange capable of possessing extremely higher throughput than the data transmission speed in the trunk line. This delay tendency is increased even more when the traffic owns the burst characteristics. In the specific condition, the data packets to be sent may overflow the buffer circuit. As a consequence, the packets which have overflowed the buffer circuit may be discarded during packet communication.
As previously described, the above-described conventional bus accessing type packet communication exchanges have the following drawbacks. That is, due to the mis-matching in the operation speeds, the amount of the data packets to be transmitted, the operation speed of which is lower than the bus operation speed, temporarily becomes excessive. As a result, these waiting data packets temporarily stored in the buffer circuit may overflow this buffer circuit and thus, the overflowed packets may be discarded during packet communication.
The present invention has been made in an attempt to solve these problems of the conventional packet communication systems, and therefore has an object to provide a load controlling system used in a packet communication exchange, capable of controlling packets which are waiting to be transmitted to a common communication medium without discarding the data packets.